The present invention relates to ground fault detection equipment and more particularly relates to a novel ground fault sensor using a cross-field magnetic path to interrupt the flow of magnetic flux in a transformer core to produce a large voltage output proportional to the amount of fault current while permitting use of a small amount of low permeability core material and relatively few secondary winding turns.
It is well-known in the art that a ground fault condition, i.e. a high impedance path from one of a pair of power lines to earth ground, may be detected by use of a differential transformer having a pair of primary windings and a secondary winding. A flow of current in a pair of power lines attached to the primary windings induces a flow of magnetic flux in the transformer core, each power line causing a flow of flux in an opposite direction. When no ground fault condition exists, each flux is of equal magnitude and opposite phase to the other; a zero net flux is coupled to the secondary winding. In the event of a ground fault condition, the current in one primary winding exceeds the current in the other primary winding and a non-zero net flux is induced in the transformer core. This non-zero flux induces a voltage in the transformer secondary, which secondary voltage is detected to initiate the rapid interruption of the current flow in the monitored circuit.
The fault current detected by a ground fault detector unit is usually of a very slowly varying nature, typically having a frequency related to the line frequency, e.g., 60 Hertz. The low frequency has required the magnetic core to have a very large physical size; the low magnitude of fault current which it is desired to detect has required the differential transformer core to be of a high permeability magnetic material and to have a secondary winding consisting of a large number, typically in excess of 1000, of turns of fine wire. The secondary winding output voltage has been relatively small, due to the slowly varying nature of the signal and the small instantaneous change in net flux caused thereby. The large mass of high permeability magnetic material is expensive, as is the cost of properly winding a continuous multi-thousand turn secondary of fine wire.
Known methods for detecting a grounded neutral line condition have utilized a separate circuit to inject a voltage across the neutral line and to sense the magnitude of a current responsive thereto.